In a known manner, a turboshaft engine comprises a compression assembly comprising an air inlet duct and at least one air compression stage, or compressor, which comprises at least one movable compressor wheel onto which the duct discharges.
Compression assemblies of this type have an aerodynamic stability limit, commonly referred to as a surge line, which in particular limits the acceleration capacity of the turboshaft engine. At low operating speeds, the aerodynamic stability limit of the compression assembly is linked to an aerodynamic overload of the first compression stage, thereby resulting in too heavy an impact of the air flow reaching the first movable wheel.
A known solution, described in the patent application FR2970508, filed by the applicant, consists in mounting a grille, referred to as a pre-rotation grille, in the air inlet duct of the turboshaft engine upstream of the first movable compressor wheel in order to reduce the impact of the air flow reaching the first movable wheel by orienting the grille in the rotational direction of the first movable wheel.
A pre-rotation grille of this type comprises orientable inlet guide vanes, referred to as variable-pitch vanes, which are mounted on a casing and are evenly distributed within the air inlet duct. The grille is set, that is to say the vanes are oriented, via a control ring, and this allows the speed of the air flow at the inlet of the movable wheel to be adjusted, so as to adapt the impact of the air flow reaching the first movable wheel.
A known arrangement of such a pre-rotation grille consists in arranging the vanes of the grille such that the pre-rotation angle of the vanes and therefore the angle of orientation of the air flow can change depending on the height in the air duct, the angle of orientation of the air flow being defined as being the relative deflection of the air flow by a vane of the pre-rotation grille at a given height of the air duct. In other words, the angle of orientation of the air flow varies with the radial distance in the air inlet duct relative to the shaft of the turboshaft engine.
FIGS. 1 to 3 are schematic cross sections of the head of an assembly of two vanes 10 of a pre-rotation grille 5 and two blades 20 of a movable compressor wheel 15 from the prior art. The consecutive vanes 10 of the grille 5 are spaced apart by a distance S1 referred to as the “pitch”. Each vane 10 has a curved cross section and defines a chord C1 between the upstream end and the downstream end of the vanes 10, that is to say between the leading edge and the trailing edge of the vane 10.
When the pre-rotation grille 5 is open at high operating speeds of the compressor, for example for a setting value of the control ring (not shown) of the pre-rotation grille 5 that is equal to 0°, the pre-rotation angle of the vanes 10 of the grille 5 is normally between values of approximately 0° at the bottom of the air duct and up to approximately 15° at the top of the air duct (relative to the axis X-X). The air flow F entering the grille is therefore deflected by an angle of orientation α1 which is close to the pre-rotation angle of the vanes and is between 0° and 15° according to the height in the air duct at an absolute speed V1 at the outlet of the grille, where the axial component (along the axis X′X) is Vz1. Such a setting of the grille 5 is used for high operating speeds of the compressor, in particular at the maximum operating speed, for example during take-off in the case of a helicopter turboshaft engine.
At low operating speeds of the compressor, as shown in FIG. 2, the grille 5 is closed at least in part in order to reduce the aerodynamic load and to increase the surge margin by moving the surge line of the compressor towards low flow rates, while moving the operating line towards high flow rates, thereby allowing a high acceleration capacity of the turboshaft engine to be obtained. In such an arrangement, the control ring (not shown) of the pre-rotation grille 5 is normally set to a value, for example approximately 65°, for which the pre-rotation angle of the vanes 10 of the grille 5 is between 65° and 80° depending on the height of the flow in the air duct.
At high operating speeds of the turboshaft engine (open grille), when the relative speed W1 of the air flow reaching the first movable wheel 15 of the compressor at the top of the air duct is high, for example such that the relative Mach number at the head of the movable wheel is greater than 1.4, the pre-rotation angle of the vanes 10 of the grille 5 should be increased beyond 15° at the top of the air duct, for example up to 20°, in order to significantly reduce the relative speed W1 of the air at the inlet of the movable wheel 15 and to thus significantly improve the efficiency of the compression.
However, in such an arrangement, when the control ring of the pre-rotation grille 5 is set to a closure value of the grille 5, for example approximately 65°, at low speeds, as shown in FIG. 2, the pre-rotation angle of the vanes reaches approximately 85° at the top of the air duct, that is to say that the air flow is deflected by an angle of orientation α1 that is close to approximately 85° by the vanes 10 in the uppermost part of the duct, in particular in the region of the distal end of the vanes 10. In such a case, the axial speed Vz1, along the axis X-X, of the air flow at the outlet of the pre-rotation grille 5 at the top of the air duct becomes so low that it may cause the blades 20 of the movable wheel 15 of the compressor to aerodynamically malfunction. In other words, the boundary layers of the air no longer hold to the shape of the head profiles of the blades 20 of the movable wheel 15, and this may cause an aerodynamic stall within the movable wheel 15, which is commonly referred to as a rotating stall, which is detrimental to the aerodynamic stability of the compressor and is therefore a drawback.
An immediate solution to overcome this drawback would be to adjust the control ring to a low value at low speeds, for example of approximately 50° or 60°, in order to close the pre-rotation grille by a slightly lesser degree and to increase the axial speed Vz1 of the air flow at the top of the air duct. However, such an adjustment would reduce the angles of orientation of the air flow for the remainder of the height of the air duct, which is a drawback.